As a typical main circuit construction for inverters, indirect AC power conversion circuits which converts a commercial AC current into a DC current via a rectifier circuit and a smoothing circuit and obtains an AC output by a voltage-type converter are generally used. Meanwhile, as a method for obtaining an AC output directly from an AC voltage, direct AC power converting apparatuses typified by matrix converters are known. In this case, since large capacitors and reactors for smoothing voltage pulsations by commercial frequencies are unnecessary, converter miniaturization can be expected and therefore the direct AC power converting apparatuses have been increasingly gaining attentions as next-generation power converting apparatuses.
Also, as a direct conversion circuit with a DC link, a circuit method and a modulation principle which have no smoothing circuit in the DC link of conventional inverters have been proposed in Document 1 (Lixiang.Weigh, Thomas. A. Lipo, “Investigation of 9-switch Dual-bridge Matrix Converter Operating under Low Output Power Factor”, U.S.A., IEEE ISA2003, vol. 1, pp. 176-181), and Document 2 (U.S. Pat. No. 6,995,992). These Documents 1 and 2 disclose that an average current (voltage) of the DC link part is controlled into a pulsating current, a converter-side current is controlled into a trapezoidal waveform, and moreover inverter-side PWM (Pulse Width Modulation) is synchronized with the converter side, thus making it possible to achieve direct conversion from a commercial AC current into an AC output without generating a certain DC voltage.
In such a direct conversion circuit with the DC link, since no regenerative function is provided on the converter side, a clamp circuit of absorbing electric power is necessitated for the DC link part.
In the direct conversion circuit with the DC link part, since one AC capacitor is connected to the DC link part, a √3-time larger voltage is applied to the capacitor used in the input-side LC filter circuit, involving the use of a capacitor having a high withstand voltage. This leads to an increase in cost as a problem.
Further in the direct conversion circuit with the DC link, since the capacitor in the clamp circuit is discharged, an active element and a control circuit are necessitated, resulting in a complicated circuit construction.